That was explained in the stream. It wouldn't matter if you equipped all white gear, you'd still start TTK at the max level your armour in your inventory will allow. So, even if you dropped to level 14 with green/white armour, yet had legendary and exotic armour in your inventory or vault that would make you a 34, then you'll be a 34 to begin TTK at. You could delete all of your exotic and legendary armour now though if you really want to go for the grind, because if you still have exotics when TTK goes live, they generate "blueprints" which go straight into cyber storage and can be collected again or purchased as yr2 versions. Anyways, all who get TTK are given a boost that will buff straight to level 25.